Transition state endergonic reaction

How does the Hammond postulate apply to electrophilic addition reactions The formation of a catbocation by protonation of an alkene is an endergonic step. Thus, the transition state for alkene protonation structurally resembles the... 

Markovnikov s rule can be restated by saying that, in the addition of HX to an aikene, the more stable carbocation intermediate is formed. This result is explained by the Hammond postulate, which says that the transition state of an exergonic reaction step structurally resembles the reactant, whereas the transition state of an endergonic reaction step structurally resembles the product. Since an aikene protonation step is endergonic, the stability of the more highly substituted carbocation is reflected in the stability of the transition state leading to its formation. 

Hammond postulate (Section 6.10) A postulate stating that we can get a picture of what a given transition state looks like by looking at the structure of the nearest stable species. Exergonic reactions have transition states that resemble reactant endergonic reactions have transition states that resemble product. 

For an endergonic reaction, in which the products are at higher energy than the reactants, the energy of the transition state is closer to that of the products. Therefore, its structure is also closer to that of the products. Any bonds that are forming in the reaction are more than half formed, and any bonds that are breaking are more than half broken. 

Kf.q AG° exergonic endergonic enthalpy entropy heat of reaction exothermic endothermic bond dissociation energy reaction energy diagram transition state activation energy reaction intermediate... 

The transition state for an endergonic reaction step resembles the product of that step. 

The first step (carbocation formation) is endergonic for both reaction paths, and both transition states resemble the carbocation intermediates. Transition states for the exergonic second step also resemble the carbocation intermediate. Transition state 1 for 1-bromopentane is more like the carbocation intermediate than is transition state 1 for 2-bromopentane. 

Draw a reaction energy diagram for a one-step endergonic reaction. Label the parts of the diagram corresponding to reactanta, products, transition state. and Is ACT positive or negative ... 

As for symmetrical systems, the properties of an unsymmetrical Class I system are essentially those of the separate reactants. Although Class II systems are valence trapped, sufficiently endergonic reactions can exhibit a single minimum close to the non-interacting reactant minimum. This minimum shifts to = 0.5 only when Hah becomes very large. Provided that Hah < (>i + AG°)/2 and AG° < 2, the positions of the reactant and product minima are given by Eqs 19a and 19b, while the location of the transition state is given by Eq. 19c. 

Recently,the electron-transfer theory was extended in order to incorporate the slow and reversible chemically induced electron-exchange reactions, as observed for the fluorescer-catalyzed chemiluminescent decomposition of a-peroxylactones. It was argued that electron transfer is complete in the transition state for such a slow and irreversible endergonic electron-transfer reaction, but that the typically small slopes (— a/RT where a is about 0.3) of the In (intensity) vs. oxidation potential plot was due to the fact that only a fraction (a) of the total free-energy change manifests itself in the activation energy. 

What about the second step in rhe dectrophilic addition of HCl to an alkene—the j reaction of chloride ion with the carbocation intermediate la this step exergonicorj endergonic Does the transition state for this second step resemble the reactant (carbocation) or product (chloroaikane) Make a rough drawing of what the transition-state structure might look like. 

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